1. Field of the Invention
The present invention relates to high temperature oxide superconducting wires and methods of manufacturing such wires, and more particularly to a high temperature oxide superconducting wire and a method of manufacturing thereof used in such applications as electric power, transportation, high energy, and medical treatment.
2. Description of the Background Art
It has been reported recently that a sintered body of oxide exhibits superconductivity at a high critical temperature, and the technique for superconduction employing such a superconductor is being into practical use. The yttrium-based oxide and the bismuth-based oxide are reported respectively to exhibit the phenomenon of superconduction at 90K and 110K. Such oxide superconductors are expected to be into practical use since they exhibit superconducting properties in liquid nitrogen which is available at a relatively low cost.
In order to transfer alternating current for supplying power, for example, through such superconductors, a superconducting wire formed by coating a superconductor with a silver sheath, coating the silver sheath with an insulator (high-resistant body) and further coating the insulator with metal is employed.
The superconductor is coated with the silver sheath for preventing the crystal structure of the superconductor from being destroyed in the drawing process. The silver sheath is coated with the insulator for reducing the ac loss. The insulator is coated with the metal for providing the superconducting wire with flexibility required.
Superconducting wires having such a structure as set forth above are disclosed in, for example, International Publication No. WO96/28853 and Japanese Patent Laying-Open No. 10-50152.
According to a method disclosed in WO096/28853, a superconductor is surrounded on its periphery by silver or the like, and further surrounded on its periphery by metal. The metal is then oxidized to produce an insulator formed of metal oxide between the silver and metal.
According to a method disclosed in Japanese Patent Laying-Open No. 10-50152, a superconductor is surrounded by silver, and further surrounded on its periphery by electrically resistant alloy (high-resistant body). The resistant alloy is oxidized to produce an insulating oxide between the silver and the resistant alloy.
According to those techniques disclosed in the two publications described above, the metal is oxidized to form the insulating oxide. The oxidation allows the metal to diffuse into the superconductor and accordingly the crystal structure of the superconductor changes. A problem of decrease in the critical current density of the superconducting wire then arises.